Production of silica-free alumina



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statg P e This invention relates to the production of" alumina "fromiron containing aluminiferous materials such as bauxite, clay, 'etc.,andhasfor itsbbj'ect the-provision'bf certain improvements in theprocess ofwsolubilizing alumina, as sodium aluminate, by 'sinteringwithsodium carbonate or-soda ash.

B'auxite, the, principal ore'of alumina (aluminum "oiride, A150 hasheretofore been, and is presently, the "main s'ource ofthe high gradealumina (substantially s i lica-- and ir'on-free) requiredintheelectrolytic aluminum reduction process. 1 Glayqhas long beenrecognized as" another=-possible source of high grade alumina, and"various proposals have-heretofore beenanvanced for pro "ducihgsu'chalumina from clays. ln claygalumina is combinedwith silica fsiOgenerally in'theform of a hydrous silicate of alumina (e.g., Al O L2Si0t2H O). (llays 5 further contaimvarying amounts of ot-her con-'stituents, frequentlyreferredtd as impurities, such as excess' silica;iron" compounds, e'ig, 'ferric oxide (R 0 etc. Thecopending'applicationfof Frederick Laist, Serial "Nb: 626916) filed'Decemben 7, l95 6 describes a" process 'for the production "of highgrade alumina from an ironeontain'ing clay in two stages or treatment,in the first reflwhich silica is removed and a' crude alumina-containingferric oxide is produced, an'd'in the second of' which the ferric oxideis eliminated and a l-high grade alumina -is'*produced. Basically,thefirst'stagetreatment involves extraction ofalumina from" calcinedclay by leaching with a dilute -solutionof hydrochloric, acidtoproduce aa crudeal'umina' substantially free of 1 silica but containing-ferfric-*oxide. In"the= second stage treatment the crude alumina "issolubilized as sodium aluminate (=NaAlO advantageously by thesoda-sinter process in which it is 'sintered at an elevated temperaturein the presence of sodium carbonate or soda ash (*Na CO )"-withoutalter- 'ing the ferric oxide. Thels'inter is leached 'to dissolve th'esodium aluminate and the=resi1lting pregnant solutionis separated fromthe "insoluble leach-residue consi'sting' mainly of ferric oxidefltThesodium aluminate solution is appropriately treated toprecipitatealuminum hydroxide (ARCH E), and the precipitate is calcinedto 'producesubstantially silica- 'and iron tree alumina 'suita- "-ble asfeed-to the pot line'ofan electrolytic aluminum reduction plant; a V

- The 1 present I invention is particularly concerned with ltheprocessstepsof'solub'ilizing the alumina in the aforeunentioned crudenaluminaby soda-sintering and leaching, that: may a'lso Be -advantageously? aplied in the s'olubilizing' of' other aluminiferous materials :bthesoda-sinter earideleachingprocess. Because of the theoretically coni-I-plete 'removal I ofsilica by the acid 1 extraction process, the

aforementioned crude alumina contains only-minute fiamountsl'ofsilicaadventitiouslyi presentirbutfother alumi- --niferoust'mate'rialsnamenable a to t the soda smter: solubiliz- :ingnpro'cess may cOntain-Qinadditiomto ironoxide, small aamounts cotflsilica whichrcan E be-effectively lirninated in ithepnacticeiofcthe invention; I i T L Sodiumaluminate in an aqueous solution tends to by- -Patented Aug." =2 1960Ice drolyze with precipitation of aluminum hydroxide. To the extent thatsuch hydrolysis takes place during the separation of the solution fromthe insoluble leach residue, the precipitated aluminum hydroxidewill bediscarded and lost in the insoluble leach residue. In commercial plant?practice, an interval of about two hours should be allowed for' thenecessarysolids-liquid sepa- "ratory steps and clarification 'of thesodium'aluminate solution, and an object ofthe invention is to minimizethe tendency of the solution to hydrolyzeduring that interval. Theinvention accomplishes this object bytineluding sodium hydroxide orcaustic soda (NaOH')-in the sodium aluminate solution duringleaching ofthe s'inter. From 30 to SOgrarns, and preferably about '45 grams, ofsodium hydroxide are included. per liter of sodium aluminate solution,in which the Al O concen- "tration may be fro'rn about to aboutv14t)g./l. (grams per liter).

'The addition of lime '(CaO) to the sodium aluminate "solution will notby itself sutfice to prevent hydrolysisof the sodium aluminate. However,the sodium hydroxide "for doing so may advantageously. be formed in situby leaching the sint'ered material-in the presence of lime and-sodiumcarbonatewhich react to form free sodium hydioxide and insoluble calciumcarbonate (CaCO An excess of sodium carbonate is customarily mixed withthe aluminiferous material in'making up the soda-sinter oharge and suchexcess is available for reaction with lime to provide the contemplatedamount' of" sodium hydroxide. The lime'may be introduced either with thesinter charge The calcum carbonate formed in situ during leachingby thereaction oflime and sodium carbonate is' anefiective filter aid in theseparationland clarification of the sodium aluminate solution fromtheinsoluble leach residue.

*In the practice of soda-sintering and leaching of the "sinter, thealuminiferous material, preferably ground to norninal 100-mesh (Tylerscreen series), is subjected to a sintering heat treatment in thepresence of sodium carbonate. The sodium carbonate may advantageously bemixed with the aluminife'rous material in the form of a concentratedaqueous solution recovered from the subsequent decomposition of thesodium aluminate by-carbon dioxide. Make-up sodium carbonate may beadded dry to-and mixed with-the soda-sinter charge. The sinteringoperationistpreferably carried out at a temperature within -the' rangeof 1700 and 1900 F. with a retention period 'of about one hour at thattemperature in any appropriate type of kiln, preferably provided withapug mill type feeder, cyclone dustcollector and return, and productcooler.

In thecourse-of sinter-ing substantially all of the alumina isconvertedfwithout fusion,to WHtCY-S'OillblfiSO- diumaluminate; Whiletheiron remains in the sinter as insoluble ferrieoxide. The sinte'redproduct is leached,

prefer ably with'wash 'water from subsequent clarification operations,to dissolve the sodium aluminate, which 'is se'paratedfrom' theinsoluble leach residue by filtration or the like. exothermicandleac'hing is preferably carried out ata temperature as neartheboiling point as practical, and

The reaction during Ieaching isstrongIy at least above F. lheclarified'sodium aluminate solution -is advantageously treated' withcarbon dioxide for precipitating aluminum hydroxide since sodium car-'lzio'nate isthereby re'g'eneratedjfor reuse in the soda-sinter step.The precipitatedaluminum hydroxide is dehydrated by calcination atatem'p'erature 'of'l700-2100 F. to fiptoduce: a high gradawsubstantiallysilic-aand iron-free alumina.

'The sodium aluminate produced in the soda-sinter kiln i has a highdegree of water solubility. However, the concentration of sodiumaluminate in the leach liquor is preferably maintained within the rangeof from 100 to 140 (and most preferably about 135) grams per liter of A1in solution, since the leach liquor becomes 'viscous and filters slowlywhen the sodium aluminate concentration is too high. Sodium aluminate,when dissolved, has a tendency to hydrolyze with the formation i,

and precipitation of aluminum hydroxide which would be-lost in thesucceeding filtering or other clarifying steps. of hydrolysis to occuris minimized by carrying free sodium hydroxide in the leach liquorduring leaching and during the separation of the sodium aluminatesolution from the insoluble leach residueand clarification of thesolution, With solutions having a sodiumaluminate concentration withinthe aforementioned range, from to SOgr'ams ofv free sodium hydroxide perliter of solution gives satisfactory results. The inclusion ofv such anamount of free sodium -hydroxidegin 'the leach liquor inhibitshydrolysis for a period of time up to about 2 hours, which is amplefor'separation of the sodium In accordance with'the invention, thistendency aluminate solution from the insoluble leach residue andclarification of the solution. In practice, very satisfactory resultshave been obtained by maintaining a free sodium hydroxide concentrationof about 45 g./l. (grams per liter) where the sodium aluminate solutionas separated from the insoluble leach residue contained about in theinsoluble leach residue]. Alternatively, the lime, or both lime, andsodium carbonate, may be introduced into the circuit at the leachingstep.

Aluminiferous material which is subjectedto sodasintering to solubilizethe alumina and form a sodium aluminate solution from which alumina ofpot line grade may be recovered should be substantially silica-free.This is because silica present in the aluminiferous material issolubilized along with the alumina by the sodasinter treatment, and willcarry through into the alumina product. However, it is very difficult,and frequently impossible, to prevent adventitious contamination withsilica of the aluminiferous feed to the soda-sinter process. Evenatmospheric dust, raised from the ground on a windy day, can lead tosignificant silica contamination of such material. Moreover, it issometimes desirable to subject an aluminiferous material containing asmall but significant amount of silica to soda-sintering withouthandling it specially to eliminate its silica content.

In accordance with the present invention a substantially silica-freealuminais recovered from aluminiferous material in which a small amountof "silica is present by subjecting 'such material to a sinteringtreatment with sodium carbonate in the presence of lime. During thesintering treatment, the silica combines with, lime and alumina to forma.,lime:alumina-silica complex (evi vdently a calcium aluminum-silicate)which, is insoluble to any significant extent-in water'or inv alkalinesolutions.

When-the'producfof soda-sinteringin the presence, of

' carbonate. The latter is eliminated along with iron oxide 40 lime .isleached todissolve the sodium aluminate,: any X silicaoriginally presentremains, in the form. of such complex, in the undissolved'jleachresidue.

.zWhen lime for reacting with sodium carbonate to form free sodiumhydroxide-in the leachliquor is introduced and negligible silica.

and 10% make-up Na CO liquor, only such amount of lime as is required toelir'ninate the silica present in the aluminiferous material is includedin the soda-sinter charge. Ordinarily, however, it is preferable toinclude lime in the soda-sinter charge in sufficient amount to eliminateany silica in the aluminiferous material, and to provide, by reactionwith sodium carbonate during leaching, the desired amount of free sodiumhydroxide to inhibit hydrolysis in the sodium aluminate solution. -Anadditional advantage of forming the desired free sodium hydroxide insitu, by reaction between lime and sodium carbonate, is that-theinsoluble calcium carbonate, aids filtration of the iron oxide intheleach residue; a

Following (is an, example of, the invention as applied to the treatmentof a, crude alumina produced from an Idaho clay by the acid extractiontreatment of the aforementioned Laist application, and analyzingabout.84% A1 0 about 16% Fe og, less than 0.l% residual chlorine, Intheexample the contemplated free'sodium hydroxide in theleach liquor wasprovided during leaching by excess sodium carbonate in the sodabyincluding the same amount of lime in the soda-sinter charge, but withthe practically silica-free aluminiferous material therewas noparticular point in subjecting the lime to the heating of the sinteringstep. In general, the relative proportions by weight of (1)aluminiferous material (of about 84% A1 0 and about 16% Fe O (2) sodiumcarbonate, and (3) lime in the soda-sinter charge, are about1:l.25-1.50:0.2-0.4, where both the lime and sodiumcarbonate required toprovide the contemplated free sodium hydroxide in the leach liquor areincluded in the soda-sinter charge. I I I The crudealumina was ground tonominal 100-mesh, and mixed with recycled sodium carbonate solution andmake-up sodium carbonate, in the proportion of about 40% crude alumina,50% 'Na CO in recycled solution This mixture was.sintered at atemperature of 1900" F. for 1 hour, in the course of whichthealumina wasconverted without fusion to sodium aluminate while iron remainedunchanged in the sinter as Fe O The sinter product analyzed 42.3% A1 025.6% N-a O, 23.8% Na CO (free) and 8.3% Fe, and was ground to nominalIOO-mesh. The ground sinter was leached with water (filter wash water)at a temperature of about 210 F. in about /2 hours time, and suflicientlime was added to the leaching charge to provide 45 g./l. of sodiumhydroxide in the leach liquor. The leach liquor (containing about 132g./l. A1 0 was filtered (and residue washed) at a temperature of 210 F.as rapidly as practical (less than 2 hours) in a vacuum filter. Byanalysis the filtrate (diluted with wash water) contained g./l. A1 0 57.g./l. Na O (fixed), 31 g./l. NaOH, and 10.0 g./l. Na CO (free). An A1 0recovery of from the soda-sinter product was attained. The washed solidresidue of the soda-sinter leach consisted of about 30% Fe O and 70%CaCO As a further example of the invention, if the crude alumina treatedin the foregoing example has become -leaching the sinter, separating theleach solution from the insoluble sinter residue, and treating theseparated solution to precipitate alumina, is essentially silica freeand of the requisite purity to serve as" feed to an reductionpot; ,1 I

In lieu of adding all:th e lime to the soda-sinter charge when the crudealumina contains some silica contaminatron, a portion only of thelime,at least suficient in amount to form aninsoluble complex with the silicapresent, may be added to such charge, and the remainder necessary forforming the desired amount of sodium hydroxide in the leach solution maybe added in the course of the leaching operation.

We claim:

1. The process of producing silica-free alumina from an aluminiferousraw material containing iron and only a very small amount of silicarelative to the amount of alumina which comprises preparing a mixture ofthe aluminiferous material and sufiicient sodium carbonate to formsodium aluminate with the alumina therein, incorporating in said mixturean amount of lime greatly in excess of the amount required to formdicalcium silicate with the silica therein, sintering the mixture at atemperature above about 1700 F., thereby converting all the silica andan equivalent portion of the alumina to a calcium aluminum silicatewhich is substantially completely insoluble in an alkaline aqueous leachsolution and converting essentially all the remainder of the alumina tosodium aluminate, leaching the sintered mixture with an aqueous alkalinemedium to form a sodium aluminate solution free from silica and iron,and recovering alumina from the resulting leach solution.

2. The process of producing silica-free alumina from an aluminiferousraw material containing iron and only a very small amount of silicarelative to the amount of alumina which comprises preparing a mixture ofthe aluminiferous material and sufficient sodium carbonate to formsodium aluminate with the alumina therein, incorporating in said mixturean amount of lime greatly in excess of the amount required to formdicalcium silicate with the silica therein, the amount of lime being inthe range from 0.2 to 0.4 times the amount of aluminiferous material inthe mixture, sintering the mixture at a temperature above about 1700 F.,thereby converting all the silica and an equivalent portion of thealumina to a calcium aluminum silicate which is substantially completelyinsoluble in an alkaline aqueous leach solution and convertingessentially all the remainder of thealumina to sodium aluminate,leaching the sintered mixture with an aqueous alkaline medium to form asodium aluminate solution free from silica and iron, and recoveringalumina from the resulting leach solution.

3. The process of producing silica-free alumina from an aluminiferousraw material containing iron and only a very small amount of silicarelative to the amount of alumina which comprises preparing a mixture ofthe aluminiferous material and sodium carbonate in excess of the amountrequired to form sodium aluminate with the alumina therein,incorporating in said mixture an amount of lime greatly in excess of theamount required to form dicalcium silicate with the silica therein,sintering the mixture at a temperature above about 1700 F., therebyconverting all the silica and an equivalent portion of the alumina to acalcium aluminum silicate which is substantially completely insoluble inan alkaline aque ous'leach solution and converting essentially all theremainder of the alumina to sodium aluminate, leaching the sinteredmixture with an aqueous alkaline medium to dissolve substantially all ofthe sodium aluminate in a silica-free and iron-free leach solution, theexcess amounts of sodium carbonate and lime in the mixture pr1or tosintering being suflicient to form at least about 30 grams per liter ofcaustic soda in said leach solution, and recovering aluminauncontaminated by silica from the leach solution.

4. The process of producing silica-free alumina from an aluminiferousraw material containing iron and only a very small amount of silicarelative to the amount of alumina which comprises preparing a mixture ofthe aluminiterous material and sodium carbonate in excess of the amountrequired to form sodium aluminate with the alumina therein,incorporating in said mixture an amount of lime greatly in excess of theamount required to form dicalcium silicate with the silica therein,sintering the mixture at a temperature above about 1700 F., therebyconverting all the silica and an equivalent portion of the alumina to acalcium aluminum silicate which is substantially completely insoluble inan alkaline aqueous medium and converting essentially all the remainderof the alumina to sodium aluminate, leaching the sintered mixture withan aqueous medium to dissolve substantially all of the sodium aluminatein a silica-free and iron-free leach solution, the excess amounts ofsodium carbonate and lime in the mixture prior to sintering beingsufficient to form at least about 30 grams per liter of caustic soda inthe leach solution and an equivalent amount of a precipitate of calciumcarbonate in intimate admixture with the insoluble residue, filteringthe sodium aluminate from the insoluble residue, and recovering aluminauncontaminated by silica from the filtrate.

5. The process of producing silica-free alumina irom an alurniniferousraw material containing iron and only a very small amount of silicarelative to the amount of alumina which comprises preparing a mixture ofthe aluminiferous material and sodium carbonate in excess of the amountrequired to form sodium aluminate with the alumina therein,incorporating in said mixture an amount of lime greatly in excess of theamount required to form dicalcium silicate with the silica therein, theamount of lime being in the range from 0.2 to 0.4 times the amount ofaluminiferous material in the mixture, sintering the mixture at atemperature above about 1700 F., thereby converting all the silica andan equivalent portion of the alumina to a calcium aluminum silicatewhich is substantially completely insoluble in an alkaline aqueous leachsolution and converting essentially all the remainder of the alumina tosodium aluminate, leaching the sintered mixture with an aqueous mediumto dissolve substantially all of the sodium aluminate in a silica-freeand iron-free leach solution, the excess amounts of sodium carbonate andlime in the mixture prior to sintering being suflicient to form from 30to 50 grams per liter of caustic soda in the leach solution and anequivalent amount of a precipitate of calcium carbonate in intimateadmixture with the insoluble residue, filtering the sodium aluminatefrom the insoluble residue, and recovering alumina uncontaminated bysilica from the filtrate.

References Cited in the file of this patent UNITED STATES PATENTS2,141,132 Folger Dec. 20, 1938 2,375,342 Brown May 8, 1945 2,398,425Half Apr. 16, 1946 2,420,852 Archibald May 20, 1947 2,421,918 Andersonet a1 June 10, 1947 2,557,891 Porter June 19, 1951 2,559,653 Mooney July10, 1951

1. THE PROCESS OF PRODUCING SILICA-FREE ALUMINA FROM AN ALUMINIFEROUSRAW MATERIAL CONTAINING IRON AND ONLY A VERY SMALL AMOUNT OF SILICARELATIVE TO THE AMOUNT OF ALUMINA WHICH COMPRISES PREPARING A MIXTURE OFTHE ALUMINIFEROUS MATERIAL AND SUFFICIENT SODIUM CARBONATE TO FORMSODIUM ALUMINATE WITH THE ALUMINA THEREIN, INCORPORATING IN SAID MIXTUREAN AMOUNT OF LIME GREATLY IN EXCESS OF THE AMOUNT REQUIRED TO FORMDICALCIUM SILICATE WITH THE SILICA THEREIN, SINTERING THE MIXTURE AT ATEMPERATURE ABOVE ABOUT 1700*F., THEREBY CONVERTING ALL THE SILICA ANDAN EQUIVALENT PORTION OF THE ALUMINUM CALCIUM ALUMINUM SILICATE WHICH ISSUBSTANTIALLY COMPLETELY INSOLUBLE IN AN ALKALINE AQUEOUS LEACH SOLUTIONAND CONVERTING ESSENTIALLY ALL THE REMAINDER OF THE ALUMINA TO SODIUMALUMINATE, LEACHING THE SINTERED MIXTURE WITH AN AQUEOUS ALKALINE MEDIUMTO FORM A SODIUM ALUMINATE SOLUTION FREE FROM SILICA AND IRON, ANDRECOVERING ALUMINA FROM THE RESULTING LEACH SOLUTION.